Diode having high brightness and method thereof

ABSTRACT

A light emitting diode includes a transparent substrate and a GaN buffer layer on the transparent substrate. An n-GaN layer is formed on the buffer layer. An active layer is formed on the n-GaN layer. A p-GaN layer is formed on the active layer. A p-electrode is formed on the p-GaN layer and an n-electrode is formed on the n-GaN layer. A reflective layer is formed on a second side of the transparent substrate. Also, a cladding layer of AlGaN is between the p-GaN layer and the active layer.

This application is a Divisional of prior U.S. application Ser. No.09/905,969, filed Jul. 17, 2001 now U.S. Pat. No. 7,067,849.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to diodes, and more particularly, to lightemitting diodes (LEDs). Although the present invention is discussed withreference to light emitting diodes, the present invention can be used ina wide range of applications including, for example, other types ofdiodes such as laser diodes.

2. Discussion of the Related Art

Gallium-Nitride (GaN) based opto-electronic device technology hasrapidly evolved from the realm of device research and development tocommercial reality. Since they have been introduced in the market in1994, GaN-based opto-electronic devices have been considered one of themost promising semiconductor devices. The efficiency of GaN lightemitting diodes (LEDs), for example, has surpassed that of incandescentlighting, and is now comparable with that of fluorescent lighting.

The market growth for GaN based devices has been far exceeding than theindustrial market prediction every year. In some applications, such astraffic lights and interior lighting in automobiles, the low maintenancecost and reduced power consumption of GaN LED's already outweigh therelatively high manufacturing costs. In other applications such asgeneral room lighting, manufacturing costs are still much too high, anda simple economy of scale reveals that such devices are not yet thesolution. Although considerably more demanding of materials quality anddevice design, room temperature, continuous wave blue lasers withreasonable lifetimes have been demonstrated. Their continued developmentcombined with the potentially high-volume market should bring costs toacceptable levels, provided that they can be manufactured with highyield. GaN-based high-power electronic devices should also findapplication in mobile communications, another high-volume market. Inorder to expand the current AlInGaN-based LED market, it is crucial todevelop low cost processing techniques without sacrificing deviceperformances. Moreover, high power optical devices are strongly neededto replace the light bulb lamps. Accordingly, two important technicalissues need to be solved at the same time, i.e., economical deviceproduction and high output power device fabrication.

Outdoor signboard display has been one of the primary markets since theintroduction of blue LEDs. In such application, the light output isconsidered one of the most important device parameters in AlInGaN-basedLEDs. As a result, the unit device price is approximately proportionalto the light output intensity. Moreover, recently, the white LEDapplication requires higher light output than currently available toreplace the incandescent light bulbs for illumination. Therefore,developing a technology to increase light output is one of the mostimportant tasks in the AlInGaN-based opto-electronic devices.

FIG. 1 shows a conventional light emitting diode structure. Theconventional LED includes a substrate 10, such as sapphire. A bufferlayer 12 made of, for example, gallium nitride (GaN) is formed on thesubstrate 10. An n-type GaN layer 14 is formed on the buffer layer 12.An active layer such as a multiple quantum well (MQW) layer 16 ofAlInGaN, for example, is formed on the n-type GaN layer 14. A p-type GaNlayer 18 is formed on the MQW layer 16.

The MQW layer emits photons “hv” in all directions to illuminate theLED. FIG. 1 shows directions 1, 2 and 3 for convenience. Photonstraveling in directions 1 and 2 contribute to the intensity of the LED.However, photons traveling in direction 3 become absorbed by thesubstrate and the package which house the LED. This photon absorptiondecreases the light extraction efficiency resulting in reducedbrightness of the LED.

There are two main methods to increase light output of the AlInGaN-basedLEDs. The first method is to improve external quantum efficiency of theLED device by epitaxial growth and device structure design. Thistechnique requires high quality epitaxial growth techniques that includeMOCVD (Metal Organic Chemical Vapor Deposition), MBE (Molecular BeamEpitaxy), and HVPE (Hydride Vapor Phase Epitaxy) and sophisticateddevice design. In particular, MOCVD has been the most common growth toolto grow commercial grade AlInGaN-based LEDs. It is generally known thatthe epitaxial film quality is strongly dependent on the types of MOCVDgrowth method. Hence, in the manufacturing point of view, it is moredifficult to improve optical light output of the LED devices by suchgrowth technique.

Another method to enhance the optical light output is increasing thelight extraction efficiency by optimizing the LED chip design. Comparedto the method of increasing external quantum efficiency by epitaxialgrowth and device structure design, this method is much simpler andeasier to increase the light intensity of the LED device. There havebeen many attempts to design the most efficient device design. However,thus far, these attempts have not led to the level of efficiency andbrightness desired from the diode. Moreover, existing designs requirehigh manufacturing cost. Accordingly, a diode is needed that has highbrightness capability, an efficient design and low manufacturing cost.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a diode thatsubstantially obviates one or more of the problems due to limitationsand disadvantages of the related art.

An advantage of the present invention is providing a diode having highbrightness.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a lightemitting diode comprises a transparent substrate; a buffer layer on afirst surface of the transparent substrate; an n-GaN layer on the bufferlayer; an active layer on the n-GaN layer; a p-GaN layer on the activelayer; a p-electrode on the p-GaN layer; an n-electrode on the n-GaNlayer; and a reflective layer on a second side of the transparentsubstrate.

In another aspect, a method of making a light emitting diode having atransparent substrate and a buffer layer on a first surface of thetransparent substrate comprises forming an n-GaN layer on the bufferlayer; forming an active layer on the n-GaN layer; forming a p-GaN layeron the active layer; forming a p-electrode on the p-GaN layer; formingan n-electrode on the n-GaN layer; and forming a reflective layer on asecond side of the transparent substrate.

In another aspect, a method of making a light emitting diode having atransparent substrate and a buffer layer on a first surface of thetransparent substrate comprises forming an n-GaN layer on the bufferlayer; forming an active layer on the n-GaN layer; forming a p-GaN layeron the active layer; forming a p-electrode on the p-GaN layer; formingan n-electrode on the n-GaN layer; and forming a reflective layer on asecond side of the transparent substrate.

In another aspect, a method of making a light emitting diode having asubstrate comprises forming an n-type layer and a p-type layer on thesubstrate; forming an active layer between the n-type layer and thep-type layer; forming a first electrode contacting the p-type layer;forming a second electrode contacting the n-type layer; and forming areflective layer on the substrate.

In another aspect, a diode comprises a transparent substrate; an activelayer on the transparent substrate, the active layer generating photons;and a reflective layer on the transparent substrate to reflect thephotons from the active layer.

In another aspect, a method of making a diode comprises forming anactive layer over a transparent substrate, the active layer generatingphotons; and forming a reflective layer on the transparent substrate toreflect the photons from the active layer.

In another aspect, a method of making a light emitting diode having atransparent substrate comprises forming an n-GaN layer having a firstdoping concentration on a first side of the transparent substrate;forming an InGaN active layer on the n-GaN layer, the active layerhaving an In concentration in a first range; forming a p-GaN layerhaving a second doping concentration on the InGaN active layer; forminga p-type contact layer on the p-GaN layer; forming an n-type contactlayer on the n-GaN layer by etching the p-type contact layer, p-GaNlayer and the InGaN active layer; reducing a thickness of thetransparent substrate by backside lapping at a second surface of thetransparent substrate; reducing a surface roughness of the transparentsubstrate; and forming a reflective layer on a reduced surface of thetransparent substrate.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 generally shows a conventional light emitting diode;

FIGS. 2A and 2B show two different embodiments of a light emitting diodeof the present invention;

FIG. 3A-3F shows the manufacturing steps for forming the light emittingdiode of the present invention;

FIGS. 4A and 4B each show a wafer having the light emitting diodes withscribe lines;

FIG. 5 shows another embodiment of the diode of the present invention;and

FIG. 6 is a graph showing a relationship between light output andcurrent injection for an LED having a reflective layer of the presentinvention and an LED without a reflective layer.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present invention, examplesof which are illustrated in the accompanying drawings.

In order to fabricate GaN-based light emitting diodes (LEDs), sapphiresubstrate has been generally used since sapphire is very stable andrelatively cheaper. The epitaxial layer quality of the AlInGaN grown onsapphire substrate is superior to the other substrate material due totheir thermal stability and the same crystal structure of the GaN.However, there are some disadvantages in using sapphire as a substratematerial for AlInGAN-based LED device fabrication. Because the sapphireis insulator, forming an n-type bottom contact is not possible. Inaddition, it is very difficult to perform the post fabrication processesthat include the grinding, the polishing, and the scribing sincesapphire is almost as hard as diamond. However, transparent sapphiresubstrate is beneficial for the light extraction compare to the othernon-transparent compound semiconductor material such as GaAs and InP.

Nevertheless, it has not been possible to take advantage of thisimportant benefit. When sapphire is used for the substrate, p and nelectrodes should be placed on the same top electrode position. As aresult, as shown in FIG. 1, the downward photons emitted in the activeregion can suffer absorption by thick substrate and the lead frame.Hence, only photons directing top portion and edge emitting cancontribute to the optical output power. On the other hand, if areflecting surface is provided in the bottom sapphire substrate, inaddition to the top emitting and edge emitting photons, the photonsemitted to the downward direction can be reflected to the side-wall ofthe sapphire substrate or can be reflected back to the top surface. Inaddition to the backside reflective coating, the light output can beincreased by making a mirror-like or highly smooth interface between thereflective metal layer and the transparent substrate. Depending on thereflective index of the substrate material and the surface conditions,including surface roughness, there is a certain angle called an escapingangle in which the photons from the active layer reflect off of theinterface back to the substrate crystal. Therefore, at a fixedreflective index of the sapphire substrate, for example, the amount ofreflected photons can be controlled by reducing the surface roughness ofthe substrate. In the present invention, a new surface polishingtechnique is employed in addition to the conventional mechanicalpolishing techniques. An atomically flat sapphire surface was obtainedusing an inductively coupled plasma reactive ion beam etching (ICPRIE).By using ICPRIE, the sapphire surface having a surface roughness assmall as 1 nm was obtained. Moreover, the transmitted or escaped photonscan be reflected back off of the smooth surface to the substratecrystal. This results in a considerable enhancement of the total opticallight output of the LED device.

FIG. 2A illustrates an LED structure of the present invention. The lightemitting diode structure includes substrate 100, which is a transparentsubstrate, such as sapphire. The sapphire has undergone backside lappingand polishing on its back surface to maximize the light output. Prior tothe reflective metal coating, ICPRIE polishing was performed on amechanically polished sapphire substrate to further reduce the surfaceroughness. In one sample, the ICPRIE polishing process conditions wereas follows:

-   -   1600 watt RF power;    -   −350V substrate bias voltage;    -   gas mixture of 18% Cl₂/72% BCl₃/20% Ar;    -   20 degree Celsius substrate temperature;    -   40 minutes etching time; and    -   resulting etch rate was 350 nm/min, respectively.

Referring to FIG. 2A, a reflective layer 200 is on the sapphiresubstrate 100 and can be made of an aluminum mirror, for example, toreflect the photons heading toward the bottom. The reflected photonscontribute to dramatically increasing the brightness of the LED. As willbe discussed throughout the description, the material for the reflectivelayer is not limited to aluminum but may be any suitable material thatwill reflect the photons to increase the brightness of the LED.Moreover, the substrate of the LED may also be made of suitablematerials other than sapphire.

FIG. 2B illustrates another LED structure of the present invention. InFIG. 2B, the reflective layer is omitted. Although the reflective layeris omitted, the sapphire substrate 100 is polished using ICPRIE, forexample, to maximize the smoothness of the surface of the surface. Suchsmooth surface allows the photons from the active layer directed towardthe sapphire substrate to reflect off from the smooth surface of thesapphire surface to enhance the light output.

FIGS. 3A-3F illustrate the steps of making a light emitting diode, as anexample application of the present invention.

Referring to FIG. 3A, a buffer layer 120 is formed on a substrate 100.The substrate 100 is preferably made from a transparent materialincluding for example, sapphire. In addition to sapphire, the substratecan be made of zinc oxide (ZnO), gallium nitride (GaN), silicon carbide(SiC) and aluminum nitride (AlN). The buffer layer 120 is made of, forexample, GaN (Gallium Nitride) and, in this instance, the GaN was grownon the surface of the sapphire substrate 100. An n-type epitaxial layersuch as n-GaN 140 is formed on the buffer layer 120. In this instance,the n-GaN layer 140 was doped with silicon (Si) with a dopingconcentration of about 10¹⁷ cm⁻³ or greater. An active layer 160 such asan AlInGaN multiple quantum well layer is formed on the n-GaN layer 140.The active layer 160 may also be formed of a single quantum well layeror a double hetero structure. In this instance, the amount of indium(In) determines whether the diode becomes a green diode or a blue diode.For an LED having blue light, indium in the range of about 22% may beused. For an LED having green light, indium in the range of about 40%may be used. The amount of indium used may be varied depending on thedesired wavelength of the blue or green color. Subsequently, a p-GaNlayer 180 is formed on the active layer 160. In this instance, the p-GaNlayer 180 was doped with magnesium (Mg) with a doping concentration ofabout 10¹⁷ cm⁻³ or greater.

Referring to FIG. 3B, a transparent conductive layer 220 is formed onthe p-GaN layer 180. The transparent conductive layer 220 may be made ofany suitable material including, for example, Ni/Au or indium-tin-oxide(ITO). A p-type electrode 240 is then formed on one side of thetransparent conductive layer 220. The p-type electrode 240 may be madeof any suitable material including, for example, Ni/Au, Pd/Au, Pd/Ni andPt. A pad 260 is formed on the p-type electrode 240. The pad 260 may bemade of any suitable material including, for example, Au. The pad 260may have a thickness of about 5000 Å or higher.

Referring to FIG. 3C, the transparent conductive layer 220, the p-GaNlayer 180, the active layer 160 and the n-GaN layer 140 are all etchedat one portion to form an n-electrode 250 and pad 270. As shown in FIG.3C, the n-GaN layer 140 is etched partially so that the n-electrode 250may be formed on the etched surface of the n-GaN layer 140. Then-electrode 250 may be made of any suitable material including, forexample, Ti/Al, Cr/Au and Ti/Au. The pad 270 is a metal and may be madefrom the same material as the pad 260.

Referring to FIG. 3D, the thickness of the substrate 100, such as madefrom sapphire, is reduced to form a thinner substrate 100A. In thisregard, backside lapping is performed on the sapphire substrate 100 toreduce the wafer thickness. After backside lapping, mechanical polishingis performed to obtain an optically flat surface. After mechanicalpolishing, the surface roughness (Ra) may be less than about 15 nm. Suchpolishing technique can reduce the surface roughness up to about 5 nm orslightly less. Such low surface roughness adds to the reflectiveproperty of the surface.

In the present invention, the thickness of the substrate 100 can becontrolled to be in the range of, for example, 350-430 μm. Moreover, thethickness can be reduced to less than 350 μm and to less than 120 μm.Here, mechanical polishing and dry etching techniques are used. For dryetching, inductively coupled plasma (ICP) reactive ion beam etching(RIE) may be used as an example.

Referring to FIG. 3E, the surface roughness is further reduced to obtaina surface roughness of less than 1 nm. The surface roughness can bereduced from 5 nm up to less than 1 nm by using dry etching. One suchdry etching technique is inductively coupled plasma (ICP) reactive ionbeam etching (RIE) to obtain an atomically flat surface. The maximumreduction of the surface roughness further enhances the reflectivity ofthe surface. It is noted that depending on the type of material used forthe substrate 100, the surface roughness may be further reduced formaximum reflectivity of the surface.

Referring to FIG. 3F, on the polished thin substrate 100A, a reflectivematerial 200 is formed. The reflective material 200 can be any suitablematerial that can reflect light. In the present example, an aluminumcoating of about 300 nm thick was formed on the polished sapphiresurface 100A using an electron beam evaporation technique. Of course,other suitable deposition techniques may be used and differentthicknesses of the aluminum are contemplated in the present invention.Here, the aluminum may have a concentration of about 99.999% or higher,which allows the aluminum to have a mirror-like property with maximumlight reflectivity. Moreover, the reflective layer 200 entirely coversthe second side of the substrate 100A.

FIG. 5 shows an alternative embodiment in which a cladding layer 170 isformed between the p-GaN layer 180 and the active layer 160. Thecladding layer 170 is preferably formed with p-AlGaN. The cladding layer170 enhances the performance of the diode. For simplicity, FIG. 5 doesnot show the p-electrode, n-electrode and the pads.

As conceptually shown in FIGS. 2A and 2B, the photons generated in theactive layer which head toward the polished sapphire surface and thealuminum mirror coating 200 are reflected. Such reflected photons add tothe brightness of the diode (photon recovery). Adding the reflectivelayer and making atomically flat surface greatly increases thebrightness of the diode. In addition to the reflective surface of thereflective layer 200, it is important to note that the low surfaceroughness of the substrate 100 also enhances the photon reflection.

FIG. 6 is a graph showing a relationship between the light output andthe injection current of, for example, a light emitting diode (LED). Onecurve of the graph depicts an LED having a reflective layer (in thiscase, an aluminum) and the other curve depicts an LED without areflective layer. In this graph, only mechanical polishing was performedon both LED's. When the reflective aluminum layer was added to themechanically polished surface of the sapphire substrate, the lightoutput increased about 200% as compared to the device without thereflective layer.

FIG. 4A shows a wafer having LEDs formed thereon. Scribe lines 300 areformed on the wafer through the buffer layer 120 from the side havingthe LEDs (front scribing) to separate the LED chips. The scribe lines300 are formed using, for example, a dry etching technique or mechanicalscribing. The dry etching technique such as inductively coupled plasma(ICP) reactive ion beam etching (RIE) can form very narrow scribe lineson the buffer layer 120 and the substrate 100A. Using such dry etchingtechnique greatly increased the number of LED chips on the wafer becausethe space between the chips can be made very small. For example, thespace between the diode chips can be as narrow as 10 μm or lower. FIG.4B is an alternative method of forming the scribe lines in which theback side of the diode used.

The scribe lines may also be formed by a diamond stylus, which requiresa large spacing between the diode chips due to the size of the diamondstylus itself. Also, a dicing technique may be used to separate thechips.

Once the diode chips are separated, each diode may be packaged. Suchpackage may also be coated with a reflective material to further enhancethe light output.

The present invention applies a simple and inexpensive light extractionprocess to the existing device fabrication process. According to thisinvention, adding just one more step of metallization after backsidelapping and polishing allows a significant light output increase. Withfiner polishing using dry etching, in some cases, the light output canbe as much as a factor of four without a substantial increase inproduction cost.

The diode of the present invention improves light intensity of a diodesuch as an AlInGaN-based light emitting diode (LED) using a reflectivecoating. The reflective coating recovers those photons, which wouldotherwise be absorbed by the substrate or the lead frame in the LEDpackage. This increases the total external quantum efficiency of thequantum well devices. This invention can be applied not only to thecurrent commercially available blue, green, red and white LEDs but alsoto other LED devices. Using this technique, the light output wasincreased by as much as a factor of four as compared to conventional LEDdevices (without the reflective coating) without significantlysacrificing or changing other characteristics of the diode.

Although the present invention has been described in detail withreference to GaN technology diodes, the reflector and substratepolishing technique of the present invention can easily be applied toother types of diodes including red LEDs and laser diodes includingVCSELs. Although red LEDs do not use GaN, the substrate of the red LEDsmay just as easily be polished and a reflective layer can easily beattached to the polished surface of the substrate, as described above.Such technique also recovers the photons to increase the light output ofthe diode. Similar technique is also applicable for laser diodes.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the split or scope of the invention. Thus, it is intendedthat the present invention cover the modifications and variations ofthis invention provided they come within the scope of the appendedclaims and their equivalents.

1. A light emitting diode comprising: a substrate; an n-type layer on afirst surface of the substrate; an active layer on the n-type layer; ap-type layer on the active layer; a first electrode contacting thep-type layer; a second electrode contacting the n-type layer; and areflective layer on a second surface of the substrate wherein a surfaceroughness of at least one portion of an interface between the reflectivelayer and the second surface of the substrate is less than 15 nm.
 2. Thelight emitting diode according to claim 1, wherein the reflective layeris on a surface of the substrate and able to reflect photons from theactive layer.
 3. The light emitting diode according to claim 1, whereinthe reflective layer substantially entirely covers the second surface ofthe substrate.
 4. The light emitting diode according to claim 1, whereinthe reflective layer has sufficient reflectivity to reflect light fromthe active layer.
 5. The light emitting diode according to claim 1,wherein the reflective layer is on a side of the substrate facing awayfrom the active layer.
 6. The light emitting diode according to claim 1,wherein the reflective substantially entirely covers the side of thesubstrate facing away from the active layer.
 7. The light emitting diodeaccording to claim 1, wherein the reflective layer is directly on thesubstrate.
 8. The light emitting diode according to claim
 1. wherein thereflective layer includes aluminum.
 9. The light emitting diodeaccording to claim 8, wherein the aluminum has a concentration of about99.999% or higher.
 10. The light emitting diode according to claim 1,wherein the substrate includes a first transparent layer and a secondtransparent layer on the first transparent layer.
 11. The light emittingdiode according to claim 10, wherein the first transparent layerincludes sapphire and the second transparent layer includes a bufferlayer.
 12. The light emitting diode according to claim 11, wherein thebuffer layer includes GaN.
 13. The light emitting diode according toclaim 1, further comprising a package housing the light emitting diode.14. The light emitting diode according to claim 1, wherein the substrateincludes sapphire.
 15. The light emitting diode according to claim 1,wherein the substrate has a thickness of 350-430 μm.
 16. The lightemitting diode according to claim 1, wherein the substrate has athickness of less than 350 μm.
 17. The light emitting diode according toclaim 1, wherein the substrate has a thickness of less than 120 μm. 18.The light emitting diode according to claim 1, wherein the active layerincludes at least one of a quantum well layer and a double heterostructure.
 19. The light emitting diode according to claim 18, whereinthe quantum layer includes a multiple quantum layer.
 20. The lightemitting diode according to claim 1, wherein an average surfaceroughness of the interface between the reflective layer and the secondsurface of the substrate is less than 15 nm.
 21. The light emittingdiode according to claim 1, wherein the surface roughness of at leastone portion of the interface between the reflective layer and the secondsurface of the substrate is 5 nm and less.